Substrate Requirements for the Phosphoinositide Response in Rat Pancreatic Islets

The relationship between phosphatidylinositol hydrolysis and the first phase of insulin secretion has been investigated by briefly exposing rat pancreatic islets that had been prelabelled with myo-[2-3H]inositol to various agonists and antagonists of insulin secretion. The recovery of lipid-bound radioactivity progressively decreased as the D-glucose concentration of the incubation medium was increased. Those carbohydrates that stimulated insulin secretion evoked a phosphoinositide response, whereas non-stimulatory carbohydrates did not. With the notable exception of amino acids, non-carbohydrate secretagogues were also found to decrease the islet lipid-bound radioactivity. Inhibition of islet glucose metabolism was found to decrease the recovery of lipid-bound radioactivity, largely as a result of impaired de novo phosphoinositide synthesis. The phosphoinositide response to glucose was not affected by inhibition of microtubular function, but was dependent upon the availability of extracellular calcium ions. We conclude that the phosphoinositide response in carbohydrate-stimulated islets is not directly related to the activation of membrane-associated glucose receptors, but may occur as a consequence of the subsequent transmembrane movement of calcium ions. Finally, the observation that stimulatory amino acids did not evoke a phosphoinositide response suggests that, under certain circumstances, this phenomenon can be dissociated from insulin secretion.